Bottom Line:
Accurate tool tracking is a crucial task that directly affects the safety and effectiveness of many interventional medical procedures.Compared to CT and MRI, ultrasound-based tool tracking has many advantages, including low cost, safety, mobility and ease of use.We performed ex vitro and in vivo experiments, showing significant improvements of tool visualization and accurate localization using different US imaging platforms.

Affiliation: Dept. of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, MD, United States of America.

ABSTRACTAccurate tool tracking is a crucial task that directly affects the safety and effectiveness of many interventional medical procedures. Compared to CT and MRI, ultrasound-based tool tracking has many advantages, including low cost, safety, mobility and ease of use. However, surgical tools are poorly visualized in conventional ultrasound images, thus preventing effective tool tracking and guidance. Existing tracking methods have not yet provided a solution that effectively solves the tool visualization and mid-plane localization accuracy problem and fully meets the clinical requirements. In this paper, we present an active ultrasound tracking and guiding system for interventional tools. The main principle of this system is to establish a bi-directional ultrasound communication between the interventional tool and US imaging machine within the tissue. This method enables the interventional tool to generate an active ultrasound field over the original imaging ultrasound signals. By controlling the timing and amplitude of the active ultrasound field, a virtual pattern can be directly injected into the US machine B mode display. In this work, we introduce the time and frequency modulation, mid-plane detection, and arbitrary pattern injection methods. The implementation of these methods further improves the target visualization and guiding accuracy, and expands the system application beyond simple tool tracking. We performed ex vitro and in vivo experiments, showing significant improvements of tool visualization and accurate localization using different US imaging platforms. An ultrasound image mid-plane detection accuracy of ±0.3 mm and a detectable tissue depth over 8.5 cm was achieved in the experiment. The system performance is tested under different configurations and system parameters. We also report the first experiment of arbitrary pattern injection to the B mode image and its application in accurate tool tracking.

pone-0104262-g011: The trigger count versus ultrasound system settings.Catheter is parallel to the image plane. Ultrasound beams are not focused.

Mentions:
The triggering condition of the active echo system is mainly affected by two factors: the amount of acoustic power received by the active element, and the gain of the receiver. The former factor also relies on many parameters like the probe transmission power, transmission aperture, focusing, tissue attenuation, etc. An experiment is performed to investigate the working conditions of AUSPIS. In this experiment, the active echo element is aligned with the probe mid-plane in a water tank. An attenuation layer is placed between the probe and catheter to mimic tissue attenuation. The trigger count is recorded under different parameter settings. Shown in Figure 8–11, the color code indicates the trigger count per frame. Generally, to achieve the same trigger count, it requires more probe transmission power at lower receiver gain than at a higher gain. Larger transmission aperture delivers more acoustic power, thus resulting in a higher trigger count when other parameters are the same. Comparing the focusing and no focusing, the former one is relatively less sensitive to the receiver gain setting, i.e., the trigger count changes less when varying the gain. Take the figure 8 and 9 “Tx aperture = 32” as an example, the color bands between the contour lines are wider in 8a, which indicates that, within a larger range of receiver gain setting, the trigger count remains the same. This is because the focused beam has smaller beam width, when the ultrasound probe acquires a B-mode image frame, only the A-mode lines close to the element can trigger the active echo. Since the energy is spatially concentrated, less transmission power is required to trigger the active echo, so in the focused beam conditions, the functional zone (colored area) extends to lower gain area, as shown in figure 8. In the unfocused case, since the beams are wider and have more overlapping, the beams far away from the element may also trigger the echo, so the trigger count growths rapidly when increasing the gain. Similarly, smaller transmission aperture results in wider beam, the bands are narrower than that with the larger aperture. In the experiment, the data with a trigger count higher than 40 are discarded; that is why the colored area does not extend to the very high gain region in figure 8–11. Though it may be useful in the quick-tool-searching purpose, high gain with large trigger counts makes the active echo spot wide and distorted, which may result in an inaccurate tool indication in the original B-mode image.

pone-0104262-g011: The trigger count versus ultrasound system settings.Catheter is parallel to the image plane. Ultrasound beams are not focused.

Mentions:
The triggering condition of the active echo system is mainly affected by two factors: the amount of acoustic power received by the active element, and the gain of the receiver. The former factor also relies on many parameters like the probe transmission power, transmission aperture, focusing, tissue attenuation, etc. An experiment is performed to investigate the working conditions of AUSPIS. In this experiment, the active echo element is aligned with the probe mid-plane in a water tank. An attenuation layer is placed between the probe and catheter to mimic tissue attenuation. The trigger count is recorded under different parameter settings. Shown in Figure 8–11, the color code indicates the trigger count per frame. Generally, to achieve the same trigger count, it requires more probe transmission power at lower receiver gain than at a higher gain. Larger transmission aperture delivers more acoustic power, thus resulting in a higher trigger count when other parameters are the same. Comparing the focusing and no focusing, the former one is relatively less sensitive to the receiver gain setting, i.e., the trigger count changes less when varying the gain. Take the figure 8 and 9 “Tx aperture = 32” as an example, the color bands between the contour lines are wider in 8a, which indicates that, within a larger range of receiver gain setting, the trigger count remains the same. This is because the focused beam has smaller beam width, when the ultrasound probe acquires a B-mode image frame, only the A-mode lines close to the element can trigger the active echo. Since the energy is spatially concentrated, less transmission power is required to trigger the active echo, so in the focused beam conditions, the functional zone (colored area) extends to lower gain area, as shown in figure 8. In the unfocused case, since the beams are wider and have more overlapping, the beams far away from the element may also trigger the echo, so the trigger count growths rapidly when increasing the gain. Similarly, smaller transmission aperture results in wider beam, the bands are narrower than that with the larger aperture. In the experiment, the data with a trigger count higher than 40 are discarded; that is why the colored area does not extend to the very high gain region in figure 8–11. Though it may be useful in the quick-tool-searching purpose, high gain with large trigger counts makes the active echo spot wide and distorted, which may result in an inaccurate tool indication in the original B-mode image.

Bottom Line:
Accurate tool tracking is a crucial task that directly affects the safety and effectiveness of many interventional medical procedures.Compared to CT and MRI, ultrasound-based tool tracking has many advantages, including low cost, safety, mobility and ease of use.We performed ex vitro and in vivo experiments, showing significant improvements of tool visualization and accurate localization using different US imaging platforms.

Affiliation:
Dept. of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, MD, United States of America.

ABSTRACTAccurate tool tracking is a crucial task that directly affects the safety and effectiveness of many interventional medical procedures. Compared to CT and MRI, ultrasound-based tool tracking has many advantages, including low cost, safety, mobility and ease of use. However, surgical tools are poorly visualized in conventional ultrasound images, thus preventing effective tool tracking and guidance. Existing tracking methods have not yet provided a solution that effectively solves the tool visualization and mid-plane localization accuracy problem and fully meets the clinical requirements. In this paper, we present an active ultrasound tracking and guiding system for interventional tools. The main principle of this system is to establish a bi-directional ultrasound communication between the interventional tool and US imaging machine within the tissue. This method enables the interventional tool to generate an active ultrasound field over the original imaging ultrasound signals. By controlling the timing and amplitude of the active ultrasound field, a virtual pattern can be directly injected into the US machine B mode display. In this work, we introduce the time and frequency modulation, mid-plane detection, and arbitrary pattern injection methods. The implementation of these methods further improves the target visualization and guiding accuracy, and expands the system application beyond simple tool tracking. We performed ex vitro and in vivo experiments, showing significant improvements of tool visualization and accurate localization using different US imaging platforms. An ultrasound image mid-plane detection accuracy of ±0.3 mm and a detectable tissue depth over 8.5 cm was achieved in the experiment. The system performance is tested under different configurations and system parameters. We also report the first experiment of arbitrary pattern injection to the B mode image and its application in accurate tool tracking.